1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the optical scanning apparatus, and particularly to an optical scanning apparatus which is suitably applicable to laser beam printers (LBPs), digital copying machines, multi-function printers and the like that employ an electrophotographic process, and in which an optically-modulated light beam from a light source unit is reflectively deflected by a rotary polygon mirror serving as a deflecting unit, transmitted through a scanning optical system, and scanned on a surface to be scanned (a scanned surface) to record image information at a high speed. The present invention further relates to a color image forming apparatus which uses a plurality of optical scanning apparatuses, and includes a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
In a conventional optical scanning apparatus, such as a laser beam printer, image recording is carried out as follows. A light source unit emits a light beam optically modulated in accordance with an image signal, an optical deflector, such as a rotary polygon mirror, periodically deflects the light beam, an fθ lens system having fθ characteristics converges the light beam and forms a light spot on a surface of a photosensitive recording material (a photosensitive drum), and the surface of the recording material is optically scanned.
FIG. 10 schematically illustrates a main portion of a conventional optical scanning apparatus. In FIG. 10, a divergent light beam emitted from a light source unit 91 is converted into an approximately parallel light beam or a convergent light beam by a collimator lens 93, the light beam (the amount of light) is shaped by an aperture stop 92, and the light beam is incident on a cylindrical lens 94 having refractive power only in a sub scanning direction. The light beam incident on the cylindrical lens 94 emerges therefrom without any change in a main scanning section. With respect to a sub scanning section, the light beam is converged by the cylindrical lens 94, and is imaged on a place near a deflecting facet (a reflective facet) 95a of an optical deflector 95, which is comprised of a rotary polygon mirror, as an approximately linear image.
The light beam reflectively deflected by the deflecting facet 95a of the optical deflector 95 is guided onto a photosensitive drum surface serving as a scanned surface 98 through a scanning optical system (an fθ lens system) 96 having fθ characteristics. The photosensitive drum surface 98 is scanned with an imaged spot moving in a direction of an arrow B (a main scanning direction) when the optical deflector 95 is rotated in a direction of an arrow A. Image information is thus recorded.
In the above-discussed optical scanning apparatus, a BD (beam detector) sensor 35 serving as an optical detector is provided such that a start timing of image formation on the photosensitive drum surface 98 can be adjusted prior to the scanning of the photosensitive drum surface 98 with the light spot. The BD sensor 35 receives a BD light beam which is a portion of the light beam reflectively deflected by the optical deflector 95, i.e., a light beam scanning a range outside an image forming range prior to the scanning of the image forming range on the photosensitive drum surface 98. The BD light beam is reflected by a BD mirror 30, and is condensed by a BD lens (not shown) to be incident on the BD sensor 35. A BD signal (a synchronous signal) is detected from an output signal of the BD sensor 35. The start timing of image recording on the photosensitive drum surface 98 is adjusted based on the BD signal.
The scanning optical system 96 in FIG. 10 is constructed such that a conjugate relationship between the deflecting facet 95a of the optical deflector 95 and the photosensitive drum surface 98 can be established in the sub scanning section. A so-called deflecting-facet fall or inclination of the optical deflector 95 is thus compensated for.
In such an optical scanning apparatus, higher printing precision is desired in recent years, so that it is preferable to decrease unevenness of pitches between scanning lines as far as possible. Even when the scanning optical system has a function of compensating for the fall of the deflecting facet of the rotary polygon mirror, the unevenness of pitches due to the deflecting-facet fall of the rotary polygon mirror is likely to occur since positions of the light beam incident on the rotary polygon mirror differ from each other between respective scanning positions (see U.S. Pat. No. 5,245,462, and US AA2002126362).
The unevenness of pitches dz due to the deflecting-facet fall of the rotary polygon mirror is written asdz=Δ×β×γwhere γ is the amount of the deflecting-facet fall of the rotary polygon mirror, β is a magnification (a sub scanning magnification) of the scanning optical system in the sub-scanning section, and Δ is an optical path difference of the light beam incident on the rotary polygon mirror.
When the scanning optical system is to be made compact, the sub scanning magnification β increases since the scanning lens comes closer to the rotary polygon mirror. The amount of the deflecting-facet fall γ must be accordingly decreased, so that the standard of the deflecting-facet fall of the rotary polygon mirror inevitably becomes stringent.
In connection with the scanning lens, a resin mold is widely used recently, and its cost is increasingly reduced. On the other hand, the rotary polygon mirror needs to be formed of a metal material for the purposes of prevention of its deformation accompanying its rotation movement. Therefore, a ratio of the cost of the rotary polygon mirror relative to the cost of the overall scanning optical system is large.
Accordingly, as the standard of the deflecting-facet fall becomes more stringent, it becomes more difficult to reduce the cost of the rotary polygon mirror. As its solving measures, the optical path difference Δ can be decreased by reducing the size of the rotary polygon mirror. As the size of the rotary polygon mirror is reduced, the standard of the deflecting-facet fall becomes tolerant and the rotary polygon mirror can be made compact. Thus, a great reduction of the cost can be expected.
However, when the size of the rotary polygon mirror is reduced, the deflecting facet is decreased, and hence the width of a light beam passing through the scanning optical system inevitably decreases. Generally, the diameter of a spot imaged on the photosensitive drum surface can be written as(spot diameter)=a×λ×(F-number)  (A)where a is a proportional constant (about 1.64), λ is a wavelength (mm) of a light beam radiated from the light source unit, and (F-number) is an exit F-number (Fno) of the scanning optical system.
As discussed above, when the size of the rotary polygon mirror is reduced, the width of the light beam passing through the scanning optical system decreases. Accordingly, the F-number increases, and the spot diameter increases as can be understood from the relation (A). It is hence difficult to obtain a highly precise and fine image.